Answer. Were the smartphone business a normal market, the answer would be some variation of "when you feel like it, and not a day sooner." But it's not.

At most carriers, you don't pay the actual price for a phone upfront — but the hidden subsidies that inflate your bill persist after the carrier has recouped its contribution. (T-Mobile's recent switch to unsubsidized pricing represents a welcome departure.) And you can claim this discount only every 20 to 24 months, depending on the carrier.

Now new deals from AT&T, T-Mobile and Verizon Wireless blur the picture more by inviting frequent upgraders to pay more for the privilege of an early, discounted phone swap.

At AT&T, upgrading once a year, the fastest pace available, would lead you to pay almost twice the usual subsidized price for a phone (that is, 60% of its full price) without getting any break on the cost of service itself.

One answer comes from economics. If you're on a traditional, subsidized-phone plan, you can zero out your hardware costs by buying a free one- or two-year-old model as soon as you're eligible for an upgrade. Wait any longer to replace a phone, and you pay your carrier for a subsidy it's long since recouped.

To do that, open the Settings app, choose Apps, tap the undesired program and select "Disable." You will no longer see it in the home screen, the apps list or anywhere else except that app-settings screen.